Then, I see two good options. Are you likely to keep the 20 as a second tank when you get the next one? If not, get a canister and know it's overkill now but will suit your next tank. If you are likely to keep the 20, get an HOB
with a biowheel and know that it will be super easy to move to the new tank and then just as easy to resume use of the old one later.
See, my little tank has a biowheel filter. I moved the fish to their big, new home and stuck that biowheel down in the bio filter media on the 2215. Voila, instant cycle. I'm now adding fish a few at a time, no problem, no ammonia. Once I have the big tank fully stocked, I intend to restart the little tank as either a planted betta habitat or a shrimp tank. Not decided yet. But, all I have to do is get the biowheel back out of the filter and the little tank is ready to stock.